BUG/MINOR: sample: Fix 'fix_tag_value' sample when waiting for more data

The test on the return value of fix_tag_value() function was inverted. To
wait for more data, the return value must be a valid empty string and not
IST_NULL.

This patch must be backported to 2.4.

(cherry picked from commit a5aa08274241b519fcfe3077ec0cc336921392be)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed